A better understanding of the molecular basis of enzymatic breakdown and removal of lipids in lipoprotein complexes is necessary in order to elucidate the pathogenesis of disordered lipid metabolism. A lipase from human post-heparin plasma which is different from lipoprotein lipase from that source has been purified to homogeneity and a hormone-sensitive lipase from adipose tissue has been highly purified. Both of these enzymes appear to have phospholipase activity and this proposal concerns a more detailed analysis of the phospholipase activity of these two enzymes and a pure phospholipase A2 from cobra venom (Naja naja naja). These enzymes will be studied in a Triton X-100--phosphatidylcholine mixed micelle system using a "surface-as-cofactor" model previously developed to explain the action of the cobra phospholipase A2 (Deems, R. A., Eaton, B. R., and Dennis, E. A (1975) J. Biol. Chem. 250, 9013-9020). The main studies will be conducted on the cobra phospholipase because of its structural simplicity and small size (molecular weight 11,000) (Deems, R. A., and Dennis, E. A. 1975) J. Biol. Chem. 250, 9008-9012), compared to the human enzymes. These studies are aimed at developing a better understanding of the kinetic analysis of the action of phospholipases and lipolytic enzymes toward substrates in aggregated forms such as lipoprotein complexes, intracellular fat droplets, mixed micelles, and membranes. Kinetic studies on the action of the cobra enzyme toward red blood cell membranes and sonicated vesicles above and below the thermotropic phase transition as well as sequence and structural studies are also proposed.